The present invention relates to a silicon nitride porous body suitable for removing e.g. dusts or a harmful substance contained in a high temperature exhaust gas, and a process for producing it.
Cordierite honeycomb bodies have been widely used as e.g. a carrier for a catalyst which clarifies an exhaust gas of an automobile, a porous body for removing dusts in a high temperature gas containing e.g. dusts, or a porous body for removing particulates discharged from a Diesel engine.
However, although cordierite is excellent in thermal shock resistance in view of quality of material, it is not necessarily satisfactory in view of e.g. corrosion resistance and heat resistance, and accordingly silicon nitride has been attracting attention, which is excellent in corrosion resistance, heat resistance and mechanical strength.
For example, JP-A-6-256069 proposes a process of baking a molded product comprising silicon nitride particles, clay and an oxide to obtain a silicon nitride porous body, but there is a problem in view of production cost since silicon nitride particles are used as a starting material.
On the other hand, JP-A-1-188479 proposes a process of heat-treating a molded product obtained by using a mixed powder comprising metal silicon particles and silicon nitride particles as a starting material to obtain a silicon nitride porous body, but a large amount of metal silicon tends to remain since the nitriding rate is low, whereby excellent heat resistance and corrosion resistance of silicon nitride may be impaired.
Further, in view of the structure of the porous body, the size of through-holes (hereinafter referred to as cells) of conventional honeycomb bodies is limited in view of e.g. molding, and a porous body having an increased area of contact with a fluid such as gas which passes through the through-holes as flow paths is desired.
It is an object of the present invention to provide a silicon nitride porous body having an increased area of contact with a fluid such as gas which passes through through-holes as flow paths, and a process for producing it.
The present invention provides a silicon nitride porous body which is a body having a generally columnar outer shape and at least two through-holes mutually parallel to one another and which has generally perpendicularly oriented columnar crystals on the surface of the through-holes.
The present invention further provides a process for producing the above silicon nitride porous body, which comprises adding a molding aid and water to a mixture which comprises 100 parts by mass of metal silicon particles having an average particle diameter of from 1 to 150 xcexcm and from 0.2 to 45 parts by mass, as calculated as SiO2, of silicon oxide particles and wherein the total amount of the metal silicon particles and the silicon oxide particles is at least 80 mass % in the mixture, followed by kneading to obtain a kneaded product, then extrusion molding the kneaded product by means of a mold to obtain an extrusion molded product having a cross-sectional area per through-hole of from 1 to 100 mm2, and then heat-treating the extrusion molded product in nitrogen to nitride the metal silicon.
Now, the present invention will be described in detail with reference to the preferred embodiments.